The present invention relates to an arrangement and method for treating contaminated groundwater and, more particularly, to an arrangement and method for in situ bioremediation treatment of the contaminated groundwater.
Subsurface soil and groundwater, as well as ponds, lakes, streams, etc., are increasingly becoming contaminated with undesirable substances, such as hydrocarbons. The hydrocarbon contaminants often trace their source to spills of hydrocarbons, such as by oil dumping, leaks and breaks in pipes transporting such materials, or as a result of improper disposal of waste hydrocarbons. It is becoming increasingly important to remove such hydrocarbon and other contaminants from the soil and groundwater.
Conventional methods of removing contaminants from the soil and groundwater include pumping water from a well to the surface, treating the contaminated water above the surface to remove the hydrocarbon contaminants, and then returning the treated water to the subsurface region. Such method is expensive and requires a significant amount of above-ground equipment to accomplish the desired result. Another conventional method is to remove the soil with the contaminants and to backfill with uncontaminated soil. Problems with this method include disposing of the contaminated soil and resource requirements (e.g., cost).
It is becoming increasingly important to provide methods for treating contaminated water that are less expensive and require significantly less equipment because the number of contaminated sites for which treatment is needed is high. To accommodate a less expensive and less equipment-intensive requirement, in situ treatment wells have been developed that strip volatile organic compounds from the groundwater without removing the groundwater to the surface. For example, Bernhardt U.S. Pat. No. 5,116,163 illustrates such an arrangement for driving out volatile impurities from the groundwater. Bernhardt '163 teaches the use of pressurized air or other gas bubbled through the contaminated groundwater to strip volatile organic compounds from the contaminated water. The volatile organic compounds are then removed from the well with the air and treated above the surface, without removing any of the water from the well.
While this system works well to strip volatile organic compounds from the contaminated groundwater, it still requires a significant amount of above-ground treatment processes for the treatment of the removed contaminants. Removing the volatile organic compounds from the soil and groundwater merely moves the contamination problem from below ground to above ground. The contaminants are still present and pose disposal problems, and the contaminated air removed from the well is still treated by conventional means, such as passing the air through activated charcoal filtration systems. To reduce or eliminate the above-ground equipment necessary to treat the organic compounds, systems have been developed to treat these organic hydrocarbons within the well complex itself.
Processes have been developed to take advantage of native aerobic microorganisms within the soil to treat the organic compounds before they are withdrawn from the subsurface area. Such use of microorganisms to accomplish biological treatment of contaminated groundwater is illustrated in Ely et al U.S. Pat. No. 4,765,902, Hater et al U.S. Pat. No. 4,850,745, Caplin et al U.S. Pat. No. 4,992,174, Graves et al U.S. Pat. No. 5,178,491, Billings et al U.S. Pat. No. 5,221,159, and Billings et al U.S. Pat. No. 5,472,294. These patents illustrate providing nutrients and oxygen to aerobic microorganisms already present and dispersed throughout the ground in the vicinity of the contaminated groundwater or providing microorganisms from above the surface to the natural subsurface areas containing the contaminated groundwater.
These processes appear to be somewhat effective for the treatment of organic compounds that are susceptible to aerobic biological treatment. However, there is a significant body of hydrocarbons, particularly halogenated aromatic and aliphatic hydrocarbons, that are not readily susceptible to such aerobic biodegradation. Many of these compounds are susceptible to biological transformation under anaerobic or anoxic environments. Thus, provision of an anaerobic biological treatment step would enable these compounds to also be broken down and treated.
Processes have been developed to provide for anaerobic biological treatment of contaminated groundwater. Hazen et al U.S. Pat. No. 5,384,048 illustrates providing an oxygenated fluid to indigenous subsurface microorganisms to create a generally aerobic environment, but leaving pockets that are anaerobic. Payne et al U.S. Pat. No. 4,945,988 discloses providing an anaerobic area of bioremediation within the aquifer and an aerobic area of bioremediation in the vadose zone. This system utilizes native microorganisms and includes the injection of substantially oxygen-free air into the aquifer to retard formation of aerobic bacteria and the injection of oxygen-rich air into the vadose zone to stimulate growth of aerobic bacteria. This process uses native microorganisms to treat the groundwater outside of the injection points and to draw volatilized contaminants through the soil and out through withdrawal wells which terminate in the vadose zone.
Often, the compound that undergoes anaerobic biodegradation yields products that are further susceptible to biodegradation by aerobic microorganisms at a faster rate. Thus, some compounds, such as halogenated hydrocarbons, may be most effectively treated by providing for sequential anaerobic and aerobic biodegradation. Complete, sequential anaerobic and aerobic biodegradation optimally results in complete degradation to carbon dioxide and water.
Beeman U.S. Pat. No. 5,277,815 illustrates anaerobic and aerobic treatment of groundwater contaminants utilizing indigenous anaerobic and aerobic bacteria to biodegrade halogenated hydrocarbons in the subsurface aquifers. This process utilizes the natural bioreactor and indigenous microorganisms and natural flow of the groundwater and its contaminants. Once the anaerobic biodegradation proceeds to a certain point, oxygen is added to change the anaerobic conditions to aerobic conditions.
Utilization of natural bioreactors, in other words, natural circulation flow and reliance upon indigenous microorganisms, to accomplish the sequential anaerobic and aerobic bioremediation is inefficient. Also, use of natural bioreactors does not easily lend itself to the parameter control necessary for optimal removal of contaminants. Such parameters include the oxygen and nutrient content, the microorganisms concentration, temperature, and residence time within the area of bioremediation. For example, the residence time of the contaminated water within the natural anaerobic bioreactor may be insufficient to fully dehalogenate the hydrocarbons in order for aerobic bioremediation to be effective. Further, reliance upon the natural bioreactor reduces efficiency because of the possible migration of the anaerobically treated hydrocarbons away from the aerobic region, without being sufficiently treated by the natural aerobic bioreactor.
Thus, it would be desirable to provide for an in situ, sequential anaerobic, aerobic bioreactor with sufficient residence time in both the anaerobic and the aerobic bioreactors to ensure high efficiency of bioremediation of the halogenated volatile and non-volatile organic contaminants contained within the groundwater. It is also desirable to ensure that the organic contaminants, such as halogenated hydrocarbons, that are inefficiently treated by the use of aerobic microorganisms are sufficiently treated in an anaerobic bioreactor prior to being directly forwarded to an aerobic bioreactor in order to provide complete treatment of the contaminants.
Sieksmeyer et al U.S. Pat. No. 5,134,078 attempts to provide some parameter control by disclosing a method and apparatus for pumping contaminated water from a well to a surface process in which the contaminated water is transferred to a rotating biological contactor in an anaerobic environment, and then fed to another rotating biological contactor in an aerobic environment. Then the water is slowly filtered through a percolation basin sunk into the soil in order for the treated water to return to the groundwater region. Such system requires significant above-ground equipment and significant available space to locate the rotating biological contactors, the percolation pond, and the rest of the equipment associated with this system. Also, this process has the disadvantages associated with all processes in which contaminated water is removed from the well--there exists a possibility of a spillage of the contaminated water and returning the contaminants to the ground through the spilled contaminated water, and the equipment and operating costs associated with pumping the water above ground are high.
Thus, it is desirable to provide an in situ bioremediation well in which the contaminated groundwater is first pumped to an anaerobic bioreactor of known capacity and efficiency to anaerobically treat the contaminated groundwater, for example, to facilitate dehalogenation of halogenated hydrocarbons, then to provide this anaerobically pre-treated contaminated water to an aerobic bioreactor of known capacity and efficiency to aerobically treat the products of partial dehalogenation and/or partial degradation and to return directly the treated groundwater to the vicinity from which the water was originally drawn, while being able to control various parameters in both bioreactor regions to ensure efficient removal of contaminants.